Solid state imaging element and electronic device

ABSTRACT

The present disclosure relates to a solid state imaging element and an electronic device that make it possible to improve sensitivity to light on a long wavelength side. A solid state imaging element according to a first aspect of the present disclosure has a solid state imaging element in which a large number of pixels are arranged vertically and horizontally, the solid state imaging element includes a periodic concave-convex pattern on a light receiving surface and an opposite surface to the light receiving surface of a light absorbing layer as a light detecting element. The present disclosure can be applied to, for example, a CMOS and the like installed in a sensor that needs a high sensitivity to light belonging to a region on the long wavelength side, such as light in the infrared region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/312,069, filed Nov. 17, 2016, which is a U.S. National Phase ofInternational Patent Application No. PCT/JP2015/065534 filed on May 29,2015, which claims priority benefit of Japanese Patent Application No.JP 2014-120205 filed in the Japan Patent Office on Jun. 11, 2014. Eachof the above-referenced applications is hereby incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a solid state imaging element and anelectronic device, and relates particularly to a solid state imagingelement and an electronic device in which the light receivingsensitivity to light on the long wavelength side such as the infraredregion is improved.

BACKGROUND ART

Thus far, a CMOS solid state imaging element and a CCD have been knownas two-dimensional solid state imaging elements, and single-crystalsilicon (Si) is generally used for the light absorbing layer of thelight detecting element of them that performs photoelectric conversion.

Si is an indirect transition semiconductor and has a band gap of 1.1 eV,and therefore has sensitivity to visible light wavelengths tonear-infrared wavelengths of approximately 1.1 um (micrometers).However, due to the wavelength dependence of the light absorptioncoefficient, the light absorption efficiency per unit thickness becomessmaller as the wavelength becomes longer.

For example, in the case of a solid state imaging element in which thethickness of the Si layer as the light absorbing layer is 3 um, thelight absorption efficiency at a wavelength of 650 nm is approximately60 to 70%, whereas, at a wavelength of 900 nm, the light absorptionefficiency is only approximately 10 to 20% and most photons aretransmitted through the Si layer. Hence, when it is attempted to obtaina solid state imaging element having a high sensitivity to light in thered to infrared region, increasing the thickness of the Si layer isknown as an effective method.

However, increasing the thickness of the Si layer has a high degree ofmanufacturing difficulty, such as the need to perform high energyimplantation in order to obtain a desired impurity profile, andfurthermore directly leads to an increase in material cost. In addition,the ratio of the thickness to the pixel size of the solid state imagingelement is increased, and an increase in the amount of color mixingcomponents of the Si bulk in the Si layer etc. are caused; thus, this isa factor in the degradation of image quality. Moreover, an increase inthe amount of defects in the crystal etc. due to the increase in thethickness of the Si layer are factors in the degradation of pixelcharacteristics, such as an increase in dark current and white spots.

In this regard, as a method for obtaining a high sensitivity to light onthe long wavelength side without increasing the thickness of the Silayer, a structure in which the loss of light caused by an etalonphenomenon based on the interference of light is suppressed by forming afine, random concave-convex structure on the surface on the oppositeside to the light receiving surface of the pixel of the solid stateimaging element is proposed (e.g. see Patent Literature 1).

CITATION LIST Patent Literature

Patent Literature 1: WO 2010/110317 A1

SUMMARY OF INVENTION Technical Problem

In the method of Patent Literature 1, when it is used for a back-sideillumination solid state imaging element, a concave-convex structure ispatterned on the same surface as the surface on which a pixel transistorthat transfers a charge detected in the solid state imaging element etc.are arranged, and consequently there are harmful effects such as anincrease in the amount of defects in crystal planes and an increase indark current.

Furthermore, the effect of suppressing the reflected light at the lightreceiving surface is low, and the effect of suppressing the re-release,from the light receiving surface, of the light components reflected atthe surface on the opposite side to the light receiving surface is low.

The present disclosure has been made in view of such circumstances, andimproves the sensitivity to light on the long wavelength side of thesolid state imaging element without increasing the thickness of the Silayer that is the light absorbing layer.

Solution to Problem

A solid state imaging element according to an aspect of the presentdisclosure has a large number of pixels are arranged vertically andhorizontally, the solid state imaging element includes a periodicconcave-convex pattern on a light receiving surface and an oppositesurface to the light receiving surface of a light absorbing layer as alight detecting element.

The light absorbing layer may be made of single-crystal Si.

The concave-convex pattern may be formed at least on the light receivingsurface and the opposite surface to the light receiving surface of thelight absorbing layer corresponding to a pixel for IR detection out ofthe large number of pixels.

A period of the concave-convex pattern formed on the opposite surface tothe light receiving surface of the light absorbing layer may beinfinitely small.

A period of the concave-convex pattern formed on the light receivingsurface and the opposite surface to the light receiving surface of thelight absorbing layer may vary in accordance with a wavelength to besensed.

The concave-convex pattern may be formed one-dimensionally periodicallyor two-dimensionally periodically.

A crystal plane of the light receiving surface and the opposite surfaceto the light receiving surface of the light absorbing layer on which theconcave-convex pattern is formed may be a (100) plane, and a crystalplane of a wall surface of the concave-convex pattern may be a (111)plane.

A period of the concave-convex pattern may be 1 um or less.

An element isolation structure may be formed at a boundary with anadjacent pixel of the light absorbing layer.

The element isolation structure may be made of a material having arefractive index lower than a refractive index of the light absorbinglayer.

A metal reflecting wall may be formed in the element isolationstructure.

The solid state imaging element according to the first aspect of thepresent disclosure may further include a reflecting mirror structure ona lower side of the light absorbing layer.

An interconnection layer may also serve as the reflecting mirrorstructure.

An electronic device according to a second aspect of the presentdisclosure is equipped with a solid state imaging element in which alarge number of pixels are arranged vertically and horizontally. Thesolid state imaging element has a periodic concave-convex pattern on alight receiving surface and an opposite surface to the light receivingsurface of a light absorbing layer as a light detecting element.

In the first and second aspects of the present disclosure, the lightthat has entered the light absorbing layer as the light detectingelement is likely to be internally reflected due to the periodicconcave-convex pattern formed on the light receiving surface and theopposite surface to the light receiving surface, and thereby theeffective optical path length of the light absorbing layer is increased;thus, light can be absorbed with good efficiency.

Advantageous Effects of Invention

According to the first and second aspects of the present disclosure,reflection can be suppressed with good efficiency in the visible lightwavelength range to the electromagnetic wave range up to near-infraredlight, and furthermore the sensitivity of light absorption to light onthe long wavelength side can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing an example of the configurationof a solid state imaging element to which the present disclosure isapplied.

FIG. 2 is a cross-sectional view showing Modification Example 1 of thesolid state imaging element to which the present disclosure is applied.

FIG. 3 is a cross-sectional view showing Modification Example 2 of thesolid state imaging element to which the present disclosure is applied.

FIG. 4 is a cross-sectional view showing Modification Example 3 of thesolid state imaging element to which the present disclosure is applied.

FIG. 5 is a cross-sectional view showing Modification Example 4 of thesolid state imaging element to which the present disclosure is applied.

FIG. 6 is a cross-sectional view showing Modification Example 5 of thesolid state imaging element to which the present disclosure is applied.

FIG. 7 is a cross-sectional view showing Modification Example 6 of thesolid state imaging element to which the present disclosure is applied.

FIG. 8 is a cross-sectional view showing Modification Example 7 of thesolid state imaging element to which the present disclosure is applied.

FIG. 9 is a cross-sectional view showing Modification Example 8 of thesolid state imaging element to which the present disclosure is applied.

FIG. 10 is a cross-sectional view showing Modification Example 9 of thesolid state imaging element to which the present disclosure is applied.

FIG. 11 is a diagram showing examples of the structure of theconcave-convex pattern.

FIG. 12 is a cross-sectional view showing an example of pixels in whicha concave-convex pattern is used.

FIG. 13 is a cross-sectional view showing an example of pixels in whicha concave-convex pattern is used.

FIG. 14 is a cross-sectional view showing an example of pixels in whicha concave-convex pattern is used.

FIG. 15 is a cross-sectional view showing an example of pixels in whicha concave-convex pattern is used.

FIG. 16 is a cross-sectional view showing an example of pixels in whicha concave-convex pattern is used.

FIG. 17 is a diagram showing the results of simulation of the lightabsorption efficiency.

FIG. 18 is a diagram describing the restrictions on the size of theconcave-convex pattern.

FIG. 19 is a diagram describing the process of the formation of theconcave-convex pattern.

FIG. 20 is a block diagram showing an example of the configuration of anelectronic device to which the present disclosure is applied.

DESCRIPTION OF EMBODIMENT(S)

Hereinbelow, preferred embodiments of the present disclosure(hereinafter, referred to as embodiments) are described in detail withreference to the drawings.

<Example of the Configuration of the Solid State Imaging Element>

FIG. 1 is a cross-sectional view showing an example of the configurationof a back-side illumination solid state imaging element 10 that is anembodiment of the present disclosure.

The upper side of the drawing is the light receiving surface (the backsurface), and the illustration of a color filter, an on-chip lens, etc.to be arranged on the upper side of the light receiving surface isomitted. In the drawing, an example of the configuration of one pixel isshown; in a two-dimensional solid state imaging element formed of theback-side illumination solid state imaging element 10, X×Y (X and Ybeing an integer) adjacent pixels are formed on one chip, N×M (N<X, M<Y;e.g. 2×2) adjacent pixels constitute one unit, and each pixel of oneunit is configured to detect light (an electromagnetic wave) of thewavelength of any of R, G, B, and IR. This similarly applies to thesubsequent drawings.

In the configuration example of FIG. 1, a periodic (period: P), fineconcave-convex pattern 12 is formed on the light receiving surface (thesurface on the upper side of the drawing) of a Si layer 11 that is thelight absorbing layer. Similarly, a periodic (period: P′), fineconcave-convex pattern 13 is formed on the surface (the lower side inthe drawing) on the opposite side to the light receiving surface. Theconcave-convex pattern 12 can act as a good-quality anti-reflection filmhaving a low reflectance for light in a wide range from the entirevisible light wavelength range to the infrared wavelength range.Furthermore, by using a periodic structure, the increase in the surfacearea of Si can be kept finite. Thereby, dark current, random noise, theincrease in the amount of white spots, etc. due to crystal defects ofthe semiconductor crystal can be suppressed. The configuration, size,etc. of the concave-convex patterns 12 and 13 are described later.

In the configuration example of FIG. 1, an element isolation structure14 filled with a dielectric material (SiO₂ or the like) having arelatively low refractive index to Si is formed at the boundary with anadjacent pixel, that is, on the lateral side in the drawing of the Silayer 11. In the case of FIG. 1, the element isolation structure 14 isin a prism shape, and is formed by etching from the light receivingsurface side. A metal reflecting wall 15 is placed in the elementisolation structure 14.

On the lower side of the Si layer 11, a reflecting mirror structure 17made of Al, Cu, Ag, or an alloy of them is formed via an insulating film16. As the reflecting mirror structure 17, instead of forming adedicated film or layer for the purpose of the reflection of incidentlight, various interconnection layers provided on the lower side of theSi layer 11 may be made to have a function as the reflecting mirrorstructure 17. On the upper side of the Si layer 11 on which theconcave-convex pattern 12 is formed, a flattening film 18 made of SiO₂,SiN, or the like is formed.

As described above, in the case of the configuration example shown inFIG. 1, the concave-convex patterns 12 and 13 are formed on the lightreceiving surface and the opposite surface to the light receivingsurface of the Si layer 11, respectively. Further, the element isolationstructure 14 is formed at the boundary with an adjacent pixel, and thereflecting mirror structure 17 is formed on the lower side of the Silayer 11. Thereby, incident light is likely to repeat reflection in theSi layer 11, and the transmission of light from the Si layer 11 issuppressed; therefore, the light absorption efficiency in the Si layer11 can be improved. That is, the sensitivity to incident light can beincreased. Furthermore, color mixing derived from an adjacent pixel issuppressed by the element isolation structure 14 and the metalreflecting wall 15.

<Modification Example 1 of the Solid State Imaging Element>

FIG. 2 is a cross-sectional view showing another example (ModificationExample 1) of the configuration of the solid state imaging element 10that is an embodiment of the present disclosure. Components in commonwith the configuration example shown in FIG. 1 are marked with the samereference numerals, and a description thereof is omitted as appropriate.

In Modification Example 1 shown in FIG. 2, the period of theconcave-convex pattern 13 formed on the surface on the lower side of theSi layer 11 is altered to an infinitely small period.

In the case of Modification Example 1, the concave-convex patterns 12and 13 are formed on the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11, respectively.Further, the element isolation structure 14 is formed at the boundarywith an adjacent pixel, and the reflecting mirror structure 17 is formedon the lower side of the Si layer 11. Thereby, incident light is likelyto repeat reflection in the Si layer 11, and the transmission of lightfrom the Si layer 11 is suppressed; therefore, the light absorptionefficiency in the Si layer 11 can be improved. That is, the sensitivityto incident light can be increased. Furthermore, color mixing derivedfrom an adjacent pixel is suppressed by the element isolation structure14 and the metal reflecting wall 15.

<Modification Example 2 of the Solid State Imaging Element>

FIG. 3 is a cross-sectional view showing yet another example(Modification Example 2) of the configuration of the solid state imagingelement 10 that is an embodiment of the present disclosure. Componentsin common with the configuration example shown in FIG. 1 are marked withthe same reference numerals, and a description thereof is omitted asappropriate.

In Modification Example 2 shown in FIG. 3, the metal reflecting wall 15is omitted from the configuration example shown in FIG. 1.

In the case of Modification Example 2, the concave-convex patterns 12and 13 are formed on the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11, respectively.Further, the element isolation structure 14 is formed at the boundarywith an adjacent pixel, and the reflecting mirror structure 17 is formedon the lower side of the Si layer 11. Thereby, incident light is likelyto repeat reflection in the Si layer 11, and the transmission of lightfrom the Si layer 11 is suppressed; therefore, the light absorptionefficiency in the Si layer 11 can be improved. That is, the sensitivityto incident light can be increased.

<Modification Example 3 of the Solid State Imaging Element>

FIG. 4 is a cross-sectional view showing yet another example(Modification Example 3) of the configuration of the solid state imagingelement 10 that is an embodiment of the present disclosure. Componentsin common with the configuration example shown in FIG. 1 are marked withthe same reference numerals, and a description thereof is omitted asappropriate.

In Modification Example 3 shown in FIG. 4, the metal reflecting wall 15and the reflecting mirror structure 17 are omitted from theconfiguration example shown in FIG. 1.

In the case of Modification Example 3, the concave-convex patterns 12and 13 are formed on the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11, respectively.Further, the element isolation structure 14 is provided at the boundarywith an adjacent pixel. Thereby, incident light is likely to repeatreflection in the Si layer 11, and the transmission of light from the Silayer 11 is suppressed; therefore, the light absorption efficiency inthe Si layer 11 can be improved. That is, the sensitivity to incidentlight can be increased.

<Modification Example 4 of the Solid State Imaging Element>

FIG. 5 is a cross-sectional view showing yet another example(Modification Example 4) of the configuration of the solid state imagingelement 10 that is an embodiment of the present disclosure. Componentsin common with the configuration example shown in FIG. 1 are marked withthe same reference numerals, and a description thereof is omitted asappropriate.

In Modification Example 4 shown in FIG. 5, the period of theconcave-convex pattern 13 in the configuration example shown in FIG. 1is altered to an infinitely small period, and the metal reflecting wall15 is omitted.

In the case of Modification Example 4, the concave-convex patterns 12and 13 are formed on the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11, respectively.Further, the element isolation structure 14 is provided at the boundarywith an adjacent pixel. Thereby, incident light is likely to repeatreflection in the Si layer 11, and the transmission of light from the Silayer 11 is suppressed; therefore, the light absorption efficiency inthe Si layer 11 can be improved. That is, the sensitivity to incidentlight can be increased.

<Modification Example 5 of the Solid State Imaging Element>

FIG. 6 is a cross-sectional view showing yet another example(Modification Example 5) of the configuration of the solid state imagingelement 10 that is an embodiment of the present disclosure. Componentsin common with the configuration example shown in FIG. 1 are marked withthe same reference numerals, and a description thereof is omitted asappropriate.

In Modification Example 5 shown in FIG. 6, the period of theconcave-convex pattern 13 in the configuration example shown in FIG. 1is altered to an infinitely small period, and the metal reflecting wall15 and the reflecting mirror structure 17 are omitted.

In the case of Modification Example 5, the concave-convex patterns 12and 13 are formed on the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11, respectively.Further, the element isolation structure 14 is provided at the boundarywith an adjacent pixel. Thereby, incident light is likely to repeatreflection in the Si layer 11, and the transmission of light from the Silayer 11 is suppressed; therefore, the light absorption efficiency inthe Si layer 11 can be improved. That is, the sensitivity to incidentlight can be increased.

<Modification Example 6 of the Solid State Imaging Element>

FIG. 7 is a cross-sectional view showing yet another example(Modification Example 6) of the configuration of the solid state imagingelement 10 that is an embodiment of the present disclosure. Componentsin common with the configuration example shown in FIG. 1 are marked withthe same reference numerals, and a description thereof is omitted asappropriate.

In Modification Example 6 shown in FIG. 7, the period of theconcave-convex pattern 13 in the configuration example shown in FIG. 1is altered to an infinitely small period, and the metal reflecting wall15 is omitted. Further, the shape of the element isolation structure 14is altered. Specifically, the shape of the element isolation structure14 is altered to a wedge shape in which the area is reduced graduallyfrom the light receiving surface side toward the opposite side to thelight receiving surface. By forming the element isolation structure 14in a wedge shape, the ratio of the reflected light in the lateraldirection in the Si layer 11 can be further enhanced as compared to acase like the configuration example of FIG. 1 in which the shape of theelement isolation structure 14 is a prism.

In the case of Modification Example 6, the concave-convex patterns 12and 13 are formed on the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11, respectively.Further, the element isolation structure 14 in a wedge shape is providedat the boundary with an adjacent pixel. Thereby, incident light islikely to repeat reflection in the Si layer 11, and the transmission oflight from the Si layer 11 is suppressed; therefore, the lightabsorption efficiency in the Si layer 11 can be improved. That is, thesensitivity to incident light can be increased.

<Modification Example 7 of the Solid State Imaging Element>

FIG. 8 is a cross-sectional view showing yet another example(Modification Example 7) of the configuration of the solid state imagingelement 10 that is an embodiment of the present disclosure. Componentsin common with the configuration example shown in FIG. 1 are marked withthe same reference numerals, and a description thereof is omitted asappropriate.

In Modification Example 7 shown in FIG. 8, the period of theconcave-convex pattern 13 in the configuration example shown in FIG. 1is altered to an infinitely small period, and the metal reflecting wall15 is omitted. Further, the shape of the element isolation structure 14is altered to a wedge shape in which the area is reduced gradually fromthe opposite side to the light receiving surface toward the lightreceiving surface side. By forming the element isolation structure 14 ina wedge shape, the ratio of the reflected light in the lateral directionin the Si layer 11 can be further enhanced as compared to a case likethe configuration example of FIG. 1 in which the shape of the elementisolation structure 14 is a prism.

In the case of Modification Example 7, the concave-convex patterns 12and 13 are formed on the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11, respectively.Further, the element isolation structure 14 in a wedge shape is providedat the boundary with an adjacent pixel. Thereby, incident light islikely to repeat reflection in the Si layer 11, and the transmission oflight from the Si layer 11 is suppressed; therefore, the lightabsorption efficiency in the Si layer 11 can be improved. That is, thesensitivity to incident light can be increased.

<Modification Example 8 of the Solid State Imaging Element>

FIG. 9 is a cross-sectional view showing yet another example(Modification Example 8) of the configuration of the solid state imagingelement 10 that is an embodiment of the present disclosure. Componentsin common with the configuration example shown in FIG. 1 are marked withthe same reference numerals, and a description thereof is omitted asappropriate.

In Modification Example 8 shown in FIG. 9, as compared to theconfiguration example shown in FIG. 1, a moth-eye structure 21 that hasan inverted configuration to the configuration of the concave-convexpattern 12 and in which the refractive index changes step-by-step fromthe incidence side as a visual point is added to the upper side of theconcave-convex pattern 12 formed on the Si layer 11.

In the case of Modification Example 8, the moth-eye structure 21 and theconcave-convex pattern 12 are formed on the light receiving surface ofthe Si layer 11, and the concave-convex pattern 13 is formed on theopposite surface to the light receiving surface. Further, the elementisolation structure 14 is formed at the boundary with an adjacent pixel,and the reflecting mirror structure 17 is formed on the lower side ofthe Si layer 11. Thereby, incident light is likely to repeat reflectionin the Si layer 11, and the transmission of light from the Si layer 11is suppressed; therefore, the light absorption efficiency in the Silayer 11 can be improved. That is, the sensitivity to incident light canbe increased. Furthermore, color mixing derived from an adjacent pixelis suppressed by the element isolation structure 14 and the metalreflecting wall 15.

<Modification Example 9 of the Solid State Imaging Element>

FIG. 10 is a cross-sectional view showing yet another example(Modification Example 9) of the configuration of the solid state imagingelement 10 that is an embodiment of the present disclosure. Componentsin common with the configuration example shown in FIG. 1 are marked withthe same reference numerals, and a description thereof is omitted asappropriate.

In Modification Example 9 shown in FIG. 10, as compared to theconfiguration example shown in FIG. 1, an intermediate film 31 having arefractive index intermediate between the refractive indices of theflattening film 18 and the Si layer 11 is provided on the upper side ofthe concave-convex pattern 12 formed on the Si layer 11. As the materialof the intermediate film 31, a hafnium oxide film, an aluminum oxide, asilicon nitride film, or the like is used. It is preferable that theintermediate film 31 be much thinner than the depth of the concavity andhave a similar configuration to the concave-convex pattern 12. A colorfilter 32 is added to the upper side of the intermediate film 31.

In the case of Modification Example 9, the concave-convex patterns 12and 13 are formed on the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11, respectively.Further, the element isolation structure 14 is formed at the boundarywith an adjacent pixel, and the reflecting mirror structure 17 is formedon the lower side of the Si layer 11. Thereby, incident light is likelyto repeat reflection in the Si layer 11, and the transmission of lightfrom the Si layer 11 is suppressed; therefore, the light absorptionefficiency in the Si layer 11 can be improved. That is, the sensitivityto incident light can be increased. Furthermore, color mixing derivedfrom an adjacent pixel is suppressed by the element isolation structure14 and the metal reflecting wall 15. Moreover, since the color filter isformed integrally, the thickness of the solid state imaging element 10can be reduced as compared to the case where a color filter is placedseparately.

The example of the configuration of the solid state imaging element 10and Modification Examples 2 to 9 thereof described above may be combinedas appropriate.

<Examples of the Structure of the Concave-Convex Patterns 12 and 13>

Next, the structure of the concave-convex patterns 12 and 13 formed onthe light receiving surface and the opposite surface to the lightreceiving surface of the Si layer 11, respectively, is described.

FIG. 11 illustrates four examples of an inverted pyramid type, a normalpyramid type, a V-groove type, and an X-groove type as the structure ofthe concave-convex patterns 12 and 13, and shows a top view and across-sectional view of them.

The inverted pyramid type is a configuration in which a concavestructure of a quadrangular pyramid shape is formed periodically on thesurface of the Si layer 11 (the light receiving surface and the oppositesurface to the light receiving surface). The normal pyramid type is aconfiguration in which a convex structure of a quadrangular pyramidshape is formed periodically on the surface of the Si layer 11.

The V-groove type is a configuration in which straight-lined groovestructures aligned parallel to the surface of the Si layer 11 are formedperiodically. The X-groove type is a configuration in whichstraight-lined first groove structures aligned parallel to the surfaceof the Si layer 11 and straight-lined second groove structures alignedparallel to a direction orthogonal to the first trench structure areformed periodically.

The structure of the concave-convex pattern 12 and the structure of theconcave-convex pattern 13 may be of the same type, or may be ofdifferent types. The structure of the concave-convex patterns 12 and 13is not limited to the types of the four examples described above, andmay be a configuration in which an identical structure is formedperiodically.

<Examples of the Pixels in which the Concave-Convex Patterns 12 and 13are Used>

As described above, the solid state imaging element 10 of the embodimentis configured such that N×M (N<X, M<Y; e.g. 2×2) adjacent pixelsconstitute one unit, and each pixel of one unit detects light of thewavelength of any of R, G, B, and IR.

FIG. 12 shows a cross-sectional view of the pixels of one unit in thesolid state imaging element 10 of the embodiment. In the case of thedrawing, the concave-convex patterns 12 and 13 are used only for thepixel that detects IR on the longest wavelength side, where the lightabsorption efficiency is relatively low, out of the pixels that detectlight of the wavelengths of R, G, B, and IR. In the drawing, theillustration of the concave-convex pattern 13 is omitted.

FIG. 13 shows a cross-sectional view of the pixels of one unit in thesolid state imaging element 10 of the embodiment. In the case of thedrawing, the concave-convex patterns 12 and 13 are used only for thepixels that detect IR and R on the long wavelength side, where the lightabsorption efficiency is relatively low, out of the pixels that detectlight of the wavelengths of R, G, B, and IR. In the drawing, theillustration of the concave-convex pattern 13 is omitted.

FIG. 14 shows a cross-sectional view of the pixels of one unit in thesolid state imaging element 10 of the embodiment. In the case of thedrawing, the concave-convex patterns 12 and 13 are used for all thepixels that detect light of the wavelengths of R, G, B, and IR. In thedrawing, the illustration of the concave-convex pattern 13 is omitted.

FIG. 15 shows a cross-sectional view of the pixels of one unit in thesolid state imaging element 10 of the embodiment. In the case of thedrawing, a case where the concave-convex patterns 12 and 13 are used forall the pixels that detect light of the wavelengths of R, G, B, and IR,and the size (period) of the concave-convex patterns 12 and 13 varies inaccordance with the wavelength to be detected is shown. That is, theconcave-convex patterns 12 and 13 are formed such that the periodbecomes shorter from the long wavelength side, where the lightabsorption efficiency is relatively low, toward the short wavelengthside, where it is relatively high. In other words, the period of theconcave-convex patterns 12 and 13 of the pixel for IR is longest, andthe period of the concave-convex patterns 12 and 13 of the pixel for Bis shortest. In the drawing, the illustration of the concave-convexpattern 13 is omitted.

Next, FIG. 16 shows a cross-sectional view of the pixels of one unit inthe case where the solid state imaging element 10 of the embodiment isused for a front-side illumination solid state imaging element in whichvarious pixel interconnections etc. are provided on the light incidencesurface side. In the case of the drawing, the concave-convex patterns 12and 13 are used for all the pixels that detect light of the wavelengthsof R, G, B, and IR. In the drawing, the illustration of theconcave-convex pattern 13 is omitted. In the case where a mirrorstructure is placed on the back surface side of the light absorbinglayer in the front-side illumination solid state imaging element 10shown in the drawing, the mirror structure can be placed between pixelswithout a gap because the interconnection etc. are not present on theback surface side.

Although the concave-convex patterns 12 and 13 may not necessarily beformed in all the pixels of R, G, B, and IR as shown in FIG. 12 to FIG.16, the concave-convex patterns 12 and 13 are formed at least in thepixel of IR.

<Simulation of the Light Absorption Efficiency>

Next, the light absorption efficiency in the Si layer 11 of the solidstate imaging element 10 of the embodiment is described.

FIG. 17 shows the results of simulation of the change in the lightabsorption efficiency of the Si layer with respect to the wavelength ofincident light. In the drawing, the horizontal axis represents thewavelength of incident light, and the vertical axis represents the lightabsorption efficiency in the Si layer. The thickness of the Si layer isassumed to be 3 um.

In the drawing, curved line a shows the result of simulation for thecharacteristics of a conventional solid state imaging element in which aconcave-convex pattern is not formed on either surface of the Si layer(neither the light receiving surface nor the opposite surface to thelight receiving surface). Curved line b shows the result of simulationfor the characteristics of a solid state imaging element in which aconcave-convex pattern is not formed on either surface of the Si layerand a reflecting mirror structure is provided on the lower side of theSi layer. Curved lines c and d show the results of simulation for thecharacteristics of the solid state imaging element 10 of the embodiment,that is, a solid state imaging element in which the concave-convexpatterns 12 and 13 are formed on both surfaces of the Si layer 11 andthe reflecting mirror structure 17 is provided.

The characteristics shown by curved line a are that the light absorptionefficiency is lower in the entire wavelength range than those of theother curved lines, and this tendency is significant particularly on thelong wavelength side. The characteristics shown by curved line b arethat an improvement in light absorption efficiency of approximately 10%to 20% over curved line a is seen in the range of wavelengths of 650 nmto 900 nm. In curved lines c and d, an improvement in light absorptionefficiency over the characteristics of curved lines a and b is seen inthe entire wavelength range, and a significant improvement in lightabsorption efficiency is seen particularly in the range of wavelengthsof 700 nm to 900 nm corresponding to red light to infrared light.Therefore, it can be said that the solid state imaging element 10 of theembodiment has the effect of improving the light absorption efficiencyof the Si layer 11 in the case of sensing the wavelength of each of R,G, B, and IR, and has the effect of greatly improving the lightabsorption efficiency of the Si layer 11 particularly in the case ofsensing R or IR on the relatively long wavelength side.

<Restrictions on the Size of the Concave-Convex Pattern 12>

Next, the restrictions on the size of the concave-convex pattern 12 isdescribed. FIG. 18 shows the relationships of the size and the period Pof the concave-convex pattern 12.

The period P of the concave-convex pattern 12 is preferably 400/N to1500/N [nm]. N is the refractive index of the medium of the surroundingsof the concave-convex pattern 12. When N=1.5, the period P is 1000 nm=1um or less. The width W1 of the top of the opening of the concave-convexpattern 12 is set to the period P or less. The width W2 of the bottom ofthe opening of the concave-convex pattern 12 is set narrower than thewidth W1 of the top of the opening of the concave-convex pattern 12. Thespacing W3 between adjacent concavities of the concave-convex pattern 12is set to 0 or more. The depth d of the concave-convex pattern 12 is setnarrower than the width W1 of the top of the opening. Specifically,0.3·W1<d<1.0·W1, or 0.5·W1<d<0.8·W1 is satisfied.

There are also similar restrictions on the size of the concave-convexpattern 13.

<Process of the Formation of the Concave-Convex Patterns 12 and 13>

Next, the process of the formation of the concave-convex patterns 12 and13 is described. FIG. 19 shows an example of the process of theformation of the concave-convex patterns 12 and 13.

First, as shown in A of the drawing, a resist is applied to the surfaceof the Si layer 11; and next, as shown in B of the drawing, exposure isperformed by EUV exposure, electron beam lithography, or the like on theplaces where the concavities of the concave-convex pattern are to beformed; thus, marking is performed.

Next, as shown in C of the drawing, the marking places of the appliedresist are removed; as shown in D of the drawing, crystallineanisotropic etching is performed by wet etching or dry etching to formconcavities; and as shown in E of the drawing, the resist is removed.

Further, as shown in F of the drawing, the intermediate film 31 having arefractive index intermediate between the refractive indices of theflattening film 18 and the Si layer 11 is formed as a film on thesurface of the Si layer 11 including the anisotropically etchedconcavities. As the material of the intermediate film 31, a highdielectric material such as a hafnium oxide film, an aluminum oxide, ora silicon nitride film may be used. It is preferable that theintermediate film 31 be much thinner than the depth of the concavity andhave a similar configuration to the concave-convex pattern. Finally, asshown in G of the drawing, the flattening film 18 made of a dielectricmaterial is formed as a film on the upper side of the intermediate film31. As the material of the flattening film, SiO₂, SiN, or the like isused.

When forming the concavity of the concave-convex pattern, as shown in Hand I of the drawing, the light receiving surface and the oppositesurface to the light receiving surface of the Si layer 11 may be set tothe (100) crystal plane, and the wall surface of the concavity may beset to the (111) crystal plane; thereby, a high-accuracy concave-convexpattern can be formed while crystal defects are suppressed bycrystalline anisotropic etching.

<Conclusions>

By the solid state imaging element 10 of the embodiment described above,the light receiving sensitivity in the wavelength ranges of R, G, B, andIR can be improved, and particularly the light receiving sensitivity inthe IR wavelength range can be improved greatly, without increasing thethickness of the Si layer 11.

The solid state imaging element 10 that is an embodiment of the presentdisclosure can be used for either a back-side illumination type or afront-side illumination type.

The solid state imaging element 10 of the embodiment can be used for anytype of electronic device having an imaging function or a sensingfunction using a solid state imaging element, including imaging devicestypified by digital video cameras and digital still cameras.

<Example of the Configuration of the Electronic Device>

FIG. 20 shows an example of the configuration of an electronic devicefor which the solid state imaging element 10 of the embodiment is used.

An electronic device 100 illustrated has an imaging function using thesolid state imaging element 10 described above. The electronic device100 includes the solid state imaging element 10, an optical systemconfiguration unit 101, a driving unit 102, and a signal processing unit103.

The optical system configuration unit 101 is formed of an optical lensetc., and causes an optical image of a subject to be incident on thesolid state imaging element 10. The driving unit 102 generates andoutputs various timing signals related to the driving of the inside ofthe solid state imaging element 10, and thereby controls the driving ofthe solid state imaging element 10. The signal processing unit 103performs a prescribed signal processing on an image signal outputtedfrom the solid state imaging element 10, and executes processing inaccordance with the result of the signal processing. Further, the signalprocessing unit 103 outputs an image signal of the result of the signalprocessing to a later stage, and records the image signal on a recordingmedium such as a solid state memory or transfers the image signal to acertain server via a certain network, for example.

The embodiment of the present disclosure is not limited to theembodiments described above, and various alterations are possiblewithout departing from the spirit of the present disclosure.

Additionally, the present technology may also be configured as below.

(1)

A solid state imaging element in which a large number of pixels arearranged vertically and horizontally, the solid state imaging elementincluding:

a periodic concave-convex pattern on a light receiving surface and anopposite surface to the light receiving surface of a light absorbinglayer as a light detecting element.

(2)

The solid state imaging element according to (1), wherein the lightabsorbing layer is made of single-crystal Si.

(3)

The solid state imaging element according to (1) or (2), wherein theconcave-convex pattern is formed at least on the light receiving surfaceand the opposite surface to the light receiving surface of the lightabsorbing layer corresponding to a pixel for IR detection out of thelarge number of pixels.

(4)

The solid state imaging element according to any one of (1) to (3),wherein a period of the concave-convex pattern formed on the oppositesurface to the light receiving surface of the light absorbing layer isinfinitely small.

(5)

The solid state imaging element according to any one of (1) to (4),wherein a period of the concave-convex pattern formed on the lightreceiving surface and the opposite surface to the light receivingsurface of the light absorbing layer varies in accordance with awavelength to be sensed.

(6)

The solid state imaging element according to any one of (1) to (5),wherein the concave-convex pattern is formed one-dimensionallyperiodically or two-dimensionally periodically.

(7)

The solid state imaging element according to any one of (1) to (6),wherein a crystal plane of the light receiving surface and the oppositesurface to the light receiving surface of the light absorbing layer onwhich the concave-convex pattern is formed is a (100) plane, and acrystal plane of a wall surface of the concave-convex pattern is a (111)plane.

(8)

The solid state imaging element according to any one of (1) to (7),wherein a period of the concave-convex pattern is 1 um or less.

(9)

The solid state imaging element according to any one of (1) to (8),wherein an element isolation structure is formed at a boundary with anadjacent pixel of the light absorbing layer.

(10)

The solid state imaging element according to (9), wherein the elementisolation structure is made of a material having a refractive indexlower than a refractive index of the light absorbing layer.

(11)

The solid state imaging element according to (9) or (10), wherein ametal reflecting wall is formed in the element isolation structure.

(12)

The solid state imaging element according to any one of (1) to (11),further including a reflecting mirror structure on a lower side of thelight absorbing layer.

(13)

The solid state imaging element according to (12), wherein aninterconnection layer serves also as the reflecting mirror structure.

(14)

An electronic device equipped with a solid state imaging element inwhich a large number of pixels are arranged vertically and horizontally,

wherein the solid state imaging element has

a periodic concave-convex pattern on a light receiving surface and anopposite surface to the light receiving surface of a light absorbinglayer as a light detecting element.

REFERENCE SIGNS LIST

-   10 solid state imaging element-   11 Si layer-   12, 13 concave-convex pattern-   14 element isolation structure-   15 metal reflecting wall-   16 insulating film-   17 reflecting mirror structure-   18 flattening film-   31 intermediate film-   32 color filter

1. A solid state imaging element, comprising: a light detecting elementdisposed in a silicon layer; a first concave-convex pattern on a firstsurface of the silicon layer, the first surface being a light receivingsurface; and a reflecting mirror structure disposed below the lightdetecting element, wherein the reflecting mirror structure is planar ina cross sectional view, wherein the light detecting element is disposedbetween a first isolation structure and a second isolation structure,and wherein the first concave-convex pattern has an inverted pyramidshape.
 2. The solid state imaging element according to claim 1, whereinthe reflecting mirror structure is included in an interconnection layer.3. The solid state imaging element according to claim 1, wherein thelight detecting element detects infra-red (IR) light.
 4. The solid stateimaging element according to claim 1, further comprising: a secondconcave-convex pattern on a second surface of the silicon layer that isopposite the first surface.
 5. The solid state imaging element accordingto claim 4, wherein a period of the first concave-convex pattern on thefirst surface and a period of the second concave-convex pattern on thesecond surface vary based on a wavelength of light that is to be sensedby the light detecting element.
 6. The solid state imaging elementaccording to claim 4, wherein the first concave-convex pattern and thesecond periodic concave-convex pattern are one of one-dimensional ortwo-dimensional.
 7. The solid state imaging element according to claim4, wherein a first crystal plane of the first surface and the secondsurface is (100), and wherein a second crystal plane of a wall surfaceof the first concave-convex pattern is (111).
 8. The solid state imagingelement according to claim 1, wherein a period of the firstconcave-convex pattern is 1 um or less.
 9. The solid state imagingelement according to claim 1, wherein the silicon layer is made ofsingle crystal silicon.
 10. The solid state imaging element according toclaim 1, wherein the first and second isolation structures are made of amaterial of a first refractive index lower than a second refractiveindex of the light detecting element.
 11. The solid state imagingelement according to claim 1, wherein the first and second isolationstructures comprise metal reflecting walls.
 12. The solid state imagingelement according to claim 2, wherein the interconnection layer isdisposed from a first region corresponding to a first trench to a secondregion corresponding to a second trench, wherein the first trenchincludes the first isolation structure, and wherein the second trenchincludes the second isolation structure.
 13. The solid state imagingelement according to claim 1, further comprising: a secondconcave-convex pattern on a second surface of the silicon layer that isopposite the first surface, wherein, in the cross sectional view, awidth of a part of the first concave-convex pattern is different from awidth of a part of the second concave-convex pattern.
 14. The solidstate imaging element according to claim 1, wherein a widest section ofthe inverted pyramid shape period has a width that is less than or equalto a period of the first concave-convex pattern.
 15. The solid stateimaging element according to claim 14, wherein a depth of the invertedpyramid shape is less than the width of the widest section.
 16. Thesolid state imaging element according to claim 1, wherein the invertedpyramid shape has a base surface and a tip surface that are parallel tothe first surface.
 17. The solid state imaging element according toclaim 1, wherein a depth of the inverted pyramid shape is between 0.3*W1and 1.0*W1, where W1 is a width of a widest part of an opening thatdefines the inverted pyramid shape in the concave-convex pattern.
 18. Anelectronic device, comprising: a solid state imaging element, whereinthe solid state imaging element comprises: a light detecting elementdisposed in a silicon layer; a first concave-convex pattern on a firstsurface of the silicon layer, the first surface being a light receivingsurface; and a reflecting mirror structure disposed below the lightdetecting element, wherein the reflecting mirror structure is planar ina cross sectional view, wherein the light detecting element is disposedbetween a first isolation structure and a second isolation structure,and wherein the first concave-convex pattern has an inverted pyramidshape.
 19. The electronic device according to claim 18, wherein thereflecting mirror structure is included in an interconnection layer. 20.The electronic device according to claim 18, further comprising: asecond concave-convex pattern on a second surface of the silicon layerthat is opposite the first surface.